#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
	long long k,n,w;
	long long ans = 0;
	scanf("%lld %lld %lld",&k,&n,&w);
	for(int i = 1;i<=w;i++) ans += i*k;
	if(ans<=n) printf("0\n");
	else printf("%lld",ans-n);
} 
